Orthokeratology is the practice of applying rigid generally highly oxygen permeable contact lenses to the eye to reshape the anterior corneal surface, thus altering refractive error temporarily or possibly permanently. Orthokeratology is sometimes identified by other names such as corneal shaping contact lenses, overnight vision correction, overnight contact lenses to correct or control myopia, or corneal refractive therapy. Other identifiers beyond these may be utilized as well. Orthokeratology is used primarily in the treatment of myopia or myopia with astigmatism. However, orthokeratology has also been utilized, less commonly, in the refractive treatment of hyperopia or hyperopia with astigmatism.
Orthokeratology has existed in some form for many decades. Early, hard contact lenses were noted by many to temporarily alter the anterior corneal shape, thus, creating alterations in refractive error when the contact lenses were not on the eye. Early attempts to practice orthokeratology utilizing hard (PMMA or polymethyl methacrylate) contact lenses were hindered by poor technology and limited understanding of corneal physiology and the criteria necessary to predictably correct for myopia by corneal molding. Significant advances have been made in the practice of orthokeratology since the advent of corneal topography measuring instrumentation in approximately the last twenty years. The advent of computerized corneal topography has permitted the creation of contact lens designs for orthokeratology that provide better repeatability and predictability of results because the corneal topographer provides an increasingly accurate map of the surface curvature of the cornea.
Further improvements in orthokeratology occurred in recent years because rigid gas permeable contact lenses that provide a much higher oxygen permeability have demonstrated the possibility of wearing the orthokeratology lenses overnight rather than utilizing the contact lenses during the day. Further, the introduction of computer numerically controlled precision lathes and other manufacturing equipment have allowed lens designs for orthokeratology to be manufactured to higher accuracy than previously possible.
Orthokeratology has also been credited by some researchers with slowing or reducing progression of myopia in children. Results of research on this area have been mixed and the effectiveness of orthokeratology in limiting or slowing the progression of myopia is uncertain.
Because the cornea provides a large fraction of the eye's focusing power, and because of the high refractive power and steep curvature of the cornea, very small changes in the curvature of the cornea or corneal thickness can result in substantial changes in refractive error. Accordingly, in orthokeratology, specially shaped contact lenses are used to lightly press on or mold the cornea, causing the cornea to gradually change shape to correct refractive error. The corrective effect may last up to approximately 72 hours once the eye is initially adapted to the lenses. Accordingly, in a usual orthokeratology procedure, according to current practice, contact lenses are worn during sleep and removed during the day. The corneal molding effect after the lenses are removed provides a reduction or elimination of refractive error. Typically, the rigid high oxygen permeability contact lenses are worn for six to eight continuous hours during sleep. Sometimes soft contact lenses are worn during the day at least during the adaptation period for correction of residual refractive errors. Some patients experience a return of some refractive error in the evening for which the orthokeratology lenses or soft contact lenses may be worn to correct refractive error under these circumstances.
Improvements in the accuracy of corneal topography analysis has also provided additional information about the shape of the anterior cornea and has made it apparent that the optical center of the cornea does not always coincide with the visual axis or optical axis of the eye.
There is still room for improvement in the area of orthokeratology contact lenses.